Control of a thyristor of a rectifying bridge

ABSTRACT

A method and a circuit for controlling at least one thyristor constitutive of a rectifying bridge with a filtered output, consisting of closing the thyristor when the voltage thereacross becomes greater than zero, and making the gate current of the thyristor disappear when the current therein exceeds its latching current.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to the control of a thyristor usedin a rectifying bridge. The present invention more specifically relatesto thyristors used in rectifying bridges with a filtered output, thatis, where the need for turning on the thyristor is not synchronous withthe zero crossings of the A.C. supply voltage of the bridge.

[0003] 2. Discussion of the Related Art

[0004]FIG. 1 shows the electric diagram of a composite fullwaverectifying bridge of the type to which the present invention applies.More generally, the present invention applies to any filtered rectifyingbridge, whatever the number of exploited phases of the A.C. inputsignal.

[0005] Composite bridge 1 of FIG. 1 is formed of two thyristors TH1 andTH2 and of two diodes D1 and D2 connected in two parallel branches ofthe bridge between two respective positive and reference outputterminals 2 and 3. Terminals 2 and 3 are intended to provide therectified voltage which is filtered by means of a capacitor C to providea filtered D.C. supply voltage Vout to a load 4 (Q). Two input terminals5 and 6 of bridge 1 receive an A.C. voltage Vin. Terminals 5 and 6 areconnected to the respective interconnection points of the seriesassociations of the thyristors and diodes, respectively TH1-D1 andTH2-D2.

[0006] Thyristors TH1 and TH2 are controlled by a circuit 7 (CTRL)providing the signals adapted to turning on the thyristors when A.C.input voltage Vin exceeds output voltage Vout, to cause the recharge ofcapacitor C.

[0007]FIG. 2 illustrates, in a timing diagram, the operation of acomposite bridge such as illustrated in FIG. 1 with a control ofthyristors TH1 and TH2 by a constant current. In other words, in thisexample, control circuit 7 provides a constant current to the respectivegates G1 and G2 of thyristors TH1 and TH2.

[0008] In FIG. 2, output voltage Vout has been shown in full line whilerectified A.C. input voltage Vin (unfiltered) has been represented by adotted line designated as Vinr. As known, voltage Vout follows thecourse of voltage Vinr only during on periods of the rectifying bridgeto recharge capacitor C. Between these periods, capacitor C dischargesinto load 4, which decreases voltage Vout.

[0009] In the first halfwave illustrated in FIG. 2, a turning-on of thethyristors at a time t1 is assumed. Only one of thyristors TH1 or TH2conducts and recharges the capacitor until the middle of the halfwave.The second halfwave illustrated in FIG. 2 assumes a variation in thecurrent surged by the load. In this example, an increase of the load isassumed, causing a decrease of voltage Vout faster than before the firsthalfwave. In this case, time t2 of conduction of the bridge thyristorsis advanced with respect to what this conduction time (t1′) would havebeen with no modification of the load. In the second halfwave, theconducting thyristor is not the same as in the first halfwave. However,this changes nothing to the operating principles.

[0010] The example of FIG. 2 illustrates a control posing nosynchronization problem since, as soon as the voltage across one of thethyristors becomes positive, said thyristor closes instantaneously, withno current peak problem (high di/dt) at the closing.

[0011] However, according to the type of thyristor used to form thebridge, other problems are encountered.

[0012] If sensitive thyristors are used, that is, thyristors onlyrequiring a small gate current (a few tens of microamperes), to minimizelosses linked to the supply of the gate current, a parasitic triggeringproblem appears since this type of thyristor has a poor immunity againstvoltage variations thereacross.

[0013] To avoid this untimely triggering, less sensitive thyristors maybe chosen. However, the losses are then high since the thyristorrequires a gate current on the order of a few tens of milliamperes to bemade conductive. Such a gate current generates a strong reverse currentand losses of a few watts to be compared with the some ten milliwatts oflow-sensitivity thyristors.

[0014] In practice, a compromise must be made between the gate currentnecessary to trigger the thyristors and the immunity against voltagevariations thereacross.

[0015]FIG. 3 illustrates, in a timing diagram, a second conventionalexample of control of thyristors of a composite bridge. In this case,the control is a pulse control. Control circuit 9 provides, permanently,a pulse train (illustrated in FIG. 3) having a pulsewidth provided toensure a sufficient conduction (a current greater than the thyristorlatching current) before the pulse disappears. Referring to the exampleof FIG. 2, that is, in a first halfwave of rectified A.C. voltage Vinrwhere a crossing of curves Vout and Vinr occurs again at a time t1, thetriggering (closing of thyristor TH1 or TH2) is not necessarilyinstantaneous. In the example shown, time t1 is subsequent to a pulseand the beginning of the next current pulse Imp1 must thus be awaited totrigger the thyristor closing. As in FIG. 2, the second halfwave ofcurve Vinr illustrates the case of an increase in the load supplied bythe rectifying bridge. Here again, pulse Imp2 triggering the closing ofone of the thyristors may be subsequent to time t2. The maximum intervalbetween the time when curves Vout and Vinr cross and the thyristorclosing is conditioned by the pulse frequency.

[0016] Such a pulse train control enables using low-sensitivitythyristors, that is, thyristors requiring high gate currents whilelimiting reverse losses due to the absence of a constant currentsupplying the gates.

[0017] However, a major disadvantage of this solution is that itgenerates harmonic disturbances resulting from the current peaksoccurring due to the interval between times t1 and t2 and the beginningof pulses Imp1 and Imp2. The current peaks generate electromagneticdisturbances incompatible with some applications. To reduceelectromagnetic disturbances, a solution would consist of increasing thepulse frequency. However, this then increases losses since the currentbecomes closer and closer to a constant gate current.

SUMMARY OF THE INVENTION

[0018] The present invention aims at controlling the closing ofthyristors of a rectifying bridge with a filtered output which overcomesthe disadvantages of known solutions. In particular, the presentinvention aims at enabling use of sensitive thyristors, without for thisto result in high losses, nor in electromagnetic disturbances due tocurrent peaks.

[0019] The present invention also aims at providing a control circuitminimizing the current consumption for the closing of the thyristors ofa controllable rectifying bridge.

[0020] The present invention also aims at providing a solutionparticularly well adapted to the control of a composite rectifyingbridge.

[0021] To achieve these and other objects, the present inventionprovides a method for controlling at least one thyristor constitutive ofa rectifying bridge with a filtered output, consisting of:

[0022] closing the thyristor when the voltage thereacross becomesgreater than zero; and

[0023] making the gate current of the thyristor disappear when thecurrent therein exceeds its latching current.

[0024] According to an embodiment of the present invention, the voltageacross the thyristor is measured by a unidirectional resistiverectifying bridge.

[0025] According to an embodiment of the present invention, the latchingcurrent in the thyristor is detected by measuring the voltagethereacross.

[0026] The present invention also provides a circuit for controlling atleast one thyristor constitutive of a rectifying bridge with a filteredoutput, comprising:

[0027] a first comparator for controlling a circuit providing a gatecurrent to the thyristor, said comparator detecting that the voltageacross the thyristor becomes positive; and

[0028] an element for inhibiting the gate current circuit as soon as thethyristor is run through by a current greater than its latching current.

[0029] According to an embodiment of the present invention, said firstcomparator comprises a first input which receives the midpoint of aresistive dividing bridge having its terminals connected, via a diode,to the thyristor terminals, and a second input which receives a firstreference voltage.

[0030] According to an embodiment of the present invention, said firstcomparator is formed of a first bipolar transistor, the base-emittervoltage drop of which conditions said first reference voltage.

[0031] According to an embodiment of the present invention, the gatecurrent circuit is formed of a constant current source controlled by aswitch connected to the thyristor gate.

[0032] According to an embodiment of the present invention, the gatecurrent circuit comprises a second bipolar transistor having its baseconnected to the collector of the first transistor, the emitter of thesecond transistor being connected to a terminal of application of a D.C.supply voltage via a resistor and its base being connected to this D.C.supply voltage by two diodes in series.

[0033] According to an embodiment of the present invention, the circuitalso comprises:

[0034] a second comparator having an input receiving a voltageproportional to the current in the thyristor and a second inputreceiving a second reference voltage; and

[0035] a flip-flop, the respective set and reset inputs of which receivethe outputs of the first and second comparators, and the output of whichis connected to a switch for providing a gate current to the thyristor.

[0036] According to an embodiment of the present invention, the circuitcontrols several thyristors.

[0037] The present invention also provides a controllable rectifyingbridge.

[0038] The foregoing objects, features, and advantages of the presentinvention, will be discussed in detail in the following non-limitingdescription of specific embodiments in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039]FIG. 1, previously described, shows the electric diagram of acomposite bridge with a filtered output to which the present inventionapplies as an example;

[0040]FIG. 2, previously described, illustrates a first conventionalexample of control of a composite bridge;

[0041]FIG. 3, previously described, illustrates a second conventionalexample of control of a composite bridge;

[0042]FIG. 4 shows, in the form of blocks, a first embodiment of acircuit for controlling a thyristor according to the present invention;

[0043]FIGS. 5A, 5B, 5C, 5D, and 5E illustrate in the form of timingdiagrams the operation of the control circuit of FIG. 4;

[0044]FIG. 6 schematically shows in the form of blocks a secondembodiment of a thyristor control circuit according to the presentinvention;

[0045]FIGS. 7A, 7B, 7C illustrate, in the form of timing diagrams, theoperation of the circuit of FIG. 6;

[0046]FIG. 8 shows the electric diagram of an example of practicalforming of the control circuit of FIG. 6; and

[0047]FIG. 9 shows an example of application of the present invention toa fullwave composite rectifying bridge.

DETAILED DESCRIPTION

[0048] Same elements have been designated with same references in thedifferent drawings. For clarity, the timing diagrams are not to scale.Further, only those circuit elements which are necessary to theunderstanding of the present invention have been shown in the drawingsand will be described hereafter. In particular, the load supplied by arectifying bridge according to the present invention has not beendetailed and is no object of the present invention. The presentinvention applies to any load type, provided that it is supplied bymeans of the filtered output (for example, filtered by a capacitor) of arectifying element comprising a thyristor controlled according to thepresent invention.

[0049] A feature of the present invention is to cause the turning-on ofthe thyristor constitutive of a rectifying bridge with a filteredoutput, when the voltage across the thyristor becomes positive, and tomake the gate current of the thyristor disappear when the currentflowing therethrough exceeds a value chosen to be greater than itslatching current, that is, greater than the current value from which thethyristor remains on even in case of a disappearing of the gate current.

[0050]FIG. 4 shows a first embodiment of a control circuit according tothe present invention. In FIG. 4, a single thyristor TH has been shownbetween terminals A and K to be compared, in the circuit of FIG. 1, withterminals 5 and 2 or with terminals 6 and 2. This thyristor thus is, forexample, any one of thyristors TH1 or TH2 of a composite rectifyingbridge such as that in FIG. 1. More generally, thyristor TH belongs to arectifying bridge with a filtered output having any structure.

[0051] According to the present invention, gate G of thyristor TH isconnected, via a switch K, to a current source 10. Source 10 provides aconstant current 10.

[0052] According to the embodiment of FIG. 4, switch K is controlled byan RS-type flip-flop 11, the output (O) of which is connected to acontrol terminal of switch K and inputs S and R of which arerespectively connected to the outputs of voltage and current detectors12 and 13. Voltage detector 12 has the function of measuring voltage VAKacross thyristor TH and of detecting the 0 crossing of this voltagetowards positive values. Current detector 13 aims at measuring thecurrent in thyristor TH in the on state, to detect a current greaterthan the thyristor latching current.

[0053] In the example of FIG. 4, detector 12 is formed of a comparator121, an inverting input of which receives a reference voltage 122(Vref1) and a non-inverting input of which is connected to midpoint 123of a resistive dividing bridge formed of two resistors R1 and R2 betweenterminals A and K of the thyristor. To only detect the voltage crossingtowards positive values and thus make the resistive bridgeunidirectional, a diode D connects terminal A to a first terminal ofresistor R1, the other terminal of which is connected to midpoint 123.The output of comparator 121 is connected to the set terminal (S) offlip-flop 11.

[0054] Current detector 13 is formed of a comparator 131, an invertinginput of which receives a reference voltage 132 (Vref2) and anon-inverting input of which is connected to a first terminal of acurrent-to-voltage conversion resistor Rs, connected between cathode K′of thyristor TH and terminal K. The non-inverting input of comparator131 is thus connected to electrode K′ of thyristor TH. The output ofcomparator 131 is connected to reset terminal R of flip-flop 11.

[0055] The operation of the circuit of FIG. 4 will be discussedhereafter in relation with FIGS. 5A to 5E which shows, in the form oftiming diagrams, examples of shapes characteristic of circuit signals.

[0056]FIG. 5A shows an example of shape of output voltage Vout of afiltered rectifying bridge comprising thyristors controlled by thecircuit of FIG. 4 in load conditions similar to those illustrated inpreviously-described FIGS. 2 and 3. FIG. 5B illustrates the shape of thestate signal of the set terminal (S) of flip-flop 11. FIG. 5Cillustrates the shape of output signal O of flip-flop 11, and thus ofturn-on control of supply switch K of gate G of thyristor TH. FIG. 5Dillustrates the shape of current I in thyristor TH. FIG. 5E illustratesthe shape of the reset state signal (R) of flip-flop 11.

[0057] In FIG. 5A, the shape of an unfiltered rectified A.C. voltageVinr has been shown as in FIGS. 2 and 3. Voltage Vout shown in FIG. 5Acorresponds to voltage Vout of FIG. 1 of a composite rectifying bridge.Accordingly, the two successive halfwaves shown in FIG. 5A actuallycorrespond to conductions of respective thyristors TH1 and TH2. However,to simplify, the operation will be discussed in relation with thyristorTH of FIG. 4, it being understood that this operation occurs onehalfwave out of two for each of thyristors TH1 or TH2.

[0058] As soon as voltage Vout becomes smaller than voltage Vinr, thisactually means that voltage VAK across the thyristor of the concernedbranch of the bridge becomes positive. In FIGS. 5A to 5E, the possiblepositive circuit triggering threshold with respect to the zeroconditioned by voltage reference Vref1 of comparator 121 of FIG. 4 isneglected. Thus, as soon as the voltage across the thyristor becomespositive (time t1), the signal provided by comparator 121 switches stateand provides a high state to input S of flip-flop 11 (FIG. 5B). Sincethe thyristor is, at this time, off, no current is detected by detector13 (reset input R of flip-flop 11 is low). Accordingly, output O (FIG.5C) of the flip-flop provides a high state conditioning the turning-onof switch K.

[0059] This turning-on of switch K causes the triggering of thyristor THby the flowing of a gate current provided by current source 10. As soonas current I running through thyristor TH (FIG. 5D) becomes smaller thanthe threshold set by voltage reference Vref2, the output of comparator131 switches and provides a high state at the reset input (FIG. 5E) offlip-flop 11 (time t3). This state switching resets output signal 0 offlip-flop 11 and accordingly turns off switch K.

[0060] The triggering threshold of the current detector is chosenaccording to the latching current (IL) of thyristor TH, to ensure thatits control is cut off once its current has reached the latching currentof the thyristor.

[0061] Since the thyristor is triggered, independently from its gatecontrol, it will remain on as long as it is run through by a directcurrent, that is, as long as the voltage thereacross remains positive.In the application to a filtered rectifying bridge, this means that thethyristor remains on until rectified A.C. voltage Vinr falls back undervoltage Vout stored by the capacitor (time t5), that is, after thepassing at the top of halfwave Vinr (in fact, when the current flowingthrough thyristor TH becomes smaller than its hold current IH). At timet5, the blocking of thyristor TH causes the switching to the low stateof the output of comparator 131, since the voltage across the thyristoris no longer positive (in reality, greater than voltage Vref2, takingdividing bridge R1-R2 into account).

[0062] In the second halfwave illustrated in FIGS. 5A to 5E, the sameexamples as in previously-described FIGS. 2 and 3 have been shown, thatis, an increase in the load supplied by the rectifying bridge resultingin a faster decrease in voltage Vout. Accordingly, the time (t2) whenthe voltage across the thyristor becomes zero is advanced with respectto time t1 of the previous halfwave. From the viewpoint of the controlcircuit of FIG. 4, this changes nothing, that is, the switching of theoutput state of flip-flop 11 is advanced, which simply generates alonger conduction period of thyristor TH (FIG. 5D). The disappearing ofthe current in thyristor TH occurs, as for the first case, from the timewhen voltage Vinr falls back under voltage Vout stored by the outputvoltage filtering capacitor (not shown).

[0063] The disappearing of a high state at input S (FIG. 5B) offlip-flop 11 only occurs at a time t5 where thyristor TH turns off forlack of current flowing therethrough (in fact, a current smaller thanthe hold current). Time t5 thus is independent from the supplied loadconditions. This has however no incidence on the thyristor gate currentsince the disappearing of the high state on signal R (FIG. 5E) alsooccurs at time t5, forbidding the taking into account of the state oninput S.

[0064] An advantage of the present invention is that it enablesdetermining the best moment to inject a control current into thethyristor gate. Indeed, by measuring the voltage thereacross, currentpeaks at the turning-on of this thyristor are avoided. Accordingly,harmonic disturbances are suppressed.

[0065] Another advantage of the present invention is that by suppressingits control current from as soon as it is on, reverse current leakagesand the control circuit consumption are reduced.

[0066] Another advantage of the present invention is that it enablescontrolling low-sensitivity thyristors with little current (set bycurrent source 10). Indeed, since the thyristor triggering is detectedby means of detector 13, the necessary control current is reduced to aminimum.

[0067] According to an alternative embodiment not shown, comparator 21(or flip-flop 11) may be used to apply an external control signal (forexample, a start-up signal). For example, input S of the flip-flop mayreceive a logic combination (for example, by an AND gate) of the outputof comparator 121 and of an external start-up logic signal.

[0068]FIG. 6 shows a second embodiment of a control circuit according tothe present invention. This embodiment takes advantage of the functionalfeatures of the controlled thyristor(s) TH. It shows voltage detector12, switch K and current source 10 for supplying gate G of thethyristor. The main difference with respect to the diagram of FIG. 4 isthe suppression of current detector 13. Said detector is replaced with ajudicious choice of reference voltage Vref1, taking advantage of amemory effect intrinsic to thyristor TH due to its latching current IL.

[0069] According to this embodiment, threshold voltage Vref1 is chosento be greater than R2*VT(R2+R1), where VT represents the thresholdvoltage of thyristor TH at the considered operating point (VT, IT), thatis, voltage VAK from which it can become conductive when its gatereceives a control current IT.

[0070] In such a configuration, as soon as voltage VAK across thyristorTH exceeds value (R1+R2)Vref1/R2, comparator 121 turns on switch K andcurrent I0 is injected into the thyristor gate.

[0071] As soon as latching current IL is reached in thyristor TH,voltage VAK thereacross collapses, which causes a new switching of theoutput of comparator 121 and the turning-off of switch K.

[0072] As compared to the diagram of FIG. 4, that of FIG. 6 has theadvantage of avoiding the presence of a series current-to-voltageresistor Rs for current detector 13 as well as the presence of RSflip-flop 11.

[0073] The operation of the embodiment of FIG. 6 is illustrated in FIGS.7A to 7C which respectively show the shapes of output voltage Vout, ofgate current G, and of current I in thyristor TH in the conductivestate. As for the previous timing diagrams, the shape of unfilteredrectified voltage Vinr has been illustrated in dotted lines in FIG. 7A,and two successive halfwaves representing two supplied load conditionshave been shown.

[0074] In the first halfwave of voltage Vinr, when at time t1 voltageVout becomes smaller than voltage Vinr, voltage VAK across thyristor THbecomes positive. Due to the chosen threshold Vref1, the output ofcomparator 121 switches not immediately, but with a slight delay (timet1). This switching turns on switch K and causes the flowing of acurrent through gate G (FIG. 7B). A current thus starts flowing throughthyristor TH (FIG. 7C). When current I reaches latching current IL ofthe thyristor, voltage VAK thereacross decreases, which causes thereverse switching (time t3) of the output of comparator 121 and,accordingly, the turning-off of switch K and the disappearing of thethyristor gate current. This operation is linked to the choice ofthreshold Vref1 according to voltage VT, so that the triggeringthreshold of comparator 121 is greater than voltage VT. Thus, as soon aslatching current IL is reached, the output of comparator 121 switchesagain.

[0075] A similar operation is repeated in the second halfwave (times t2,t12, and t4). It can be seen that the closing duration of switch K isadapted to the load condition, in that if the closing is advanced due toan increase in the load supplied by the circuit, its opening time isalso advanced. Its conduction time (and thus the duration of applicationof a gate current) is thus reduced to what is strictly necessary, byautomatically adapting to the supplied load.

[0076] The delay (time t1 to t1 and t2 to t12) at the thyristorturning-on must be as short as possible to avoid occurrence ofprejudicial current peaks. Fulfilling this condition poses no problem inpractice.

[0077]FIG. 8 shows a third embodiment of a circuit for controlling athyristor TH according to the present invention. This drawing shows thesame characteristics as in FIG. 6 and details an example of practicalforming of its circuit. The current generator (source 10) is here formedof a PNP-type bipolar transistor T1 having its emitter connected, by aresistor R4, to a terminal 7 of application of a D.C. supply voltageVcc, and its base connected by two diodes in series D3 and D4 toterminal 7, the anodes of diodes D3 and D4 being directed towardsterminal 7. The collector of transistor T1 is connected to gate G ofthyristor TH. The base of transistor T1 is connected, by a resistor R3,to the collector of a second NPN-type bipolar transistor T2 performingthe function of comparator 121. The base of transistor T2 is connectedto the junction point of resistors R1 and R2. The emitter of transistorT2 is connected to terminal K (cathode of thyristor TH).

[0078] Resistor R3 has the function of biasing diodes D2 and D3 and oflimiting the current in transistor T2. The value I0 of the currentprovided by source 10 is equal to the voltage drop in a forward-biaseddiode divided by resistance R4. One of the two diodes D3 and D4 is usedto compensate for the base/emitter voltage of transistor T1.

[0079] Voltage VAK across thyristor TH for which a current is injectedin its gate is provided by relation (R1+R2)*VBEN/R2+VD, where VBENrepresents the base-emitter voltage drop of transistor T2 (of type NPN)in the on state.

[0080] Considering a practical example of a thyristor having an on-stateforward voltage VT of at most approximately 1.3 volts, and assumingequal resistances R1 and R2 and a voltage drop in forward-biased diode Don the order of 0.7 volt, the base-emitter voltage to which transistorT2 is submitted when thyristor TH conducts is on the order of 0.3volt.Accordingly, when the thyristor conducts, transistor T2 is off(insufficient base-emitter voltage) and no current flows through thethyristor gate.

[0081]FIG. 9 shows an example of application of the control circuitaccording to the present invention (in its embodiment of FIG. 8) to arectifying bridge adapted to limiting the surge current at the circuitpower-on.

[0082] The rectifying bridge is, as in FIG. 1, formed of two thyristorsTH1 and TH2 and of two diodes D1 and D2. Terminal 2, to which aredirectly connected the cathodes of thyristors TH1 and TH2, is connectedby a first terminal of a surge current limiting resistor R1. The secondterminal of resistor R1 is connected to terminals 5 and 6 of applicationof voltage Vin via two rectifying diodes D5 and D6. Thyristors TH1 andTH2 are used to short-circuit resistor R1 once a starting phase of thecircuit is over. In this starting phase, the (initial) charge ofcapacitor C is performed through a bridge formed of diodes D1, D2, D5,and D6. In the example of FIG. 9, the rectifying bridge supplies aD.C./D.C. converter 8, to the output terminals of which is connected aload 4 (Q).

[0083] According to the present invention, a circuit of the type shownin FIG. 8 is used to control both transistors TH1 and TH2. The emitterof transistor T2 is connected to cathodes K of thyristors TH1 and TH2.Dividing bridge R1-R2 is placed between the cathodes of diodes D5 and D6and common terminal K of thyristors TH1 and TH2. Diodes D5 and D6 theneach play the role of input diode D of dividing bridge R1, R2, accordingto the halfwave of A.C. input voltage Vin. Balancing resistors Req areplaced between the respective gates of thyristors TH1 and TH2 and thecollector of transistor T1. The function of these resistors is tobalance the gate currents of thyristors TH1 and TH2, due to possibletechnological dispersions influencing the gate-cathode voltage.

[0084] The operation of the surge current limiting circuit of FIG. 9 canbe deduced from the operations discussed in relation with the abovedrawings.

[0085] An advantage of the present invention is that it enables usinglow-sensitivity thyristors, thus exhibiting a strong immunity againststatic voltage variations, with a very low average control current,without generating additional losses linked to the reverse leakagecurrent.

[0086] Of course, the present invention is likely to have variousalterations, modifications, and improvements which will readily occur tothose skilled in the art. In particular, the sizing of the voltagethresholds and of the resistors will be chosen according to thecharacteristics of the thyristors used. Further, although the presentinvention has been described hereabove in relation with a fullwavecomposite bridge, it may be implemented in any rectifying structureusing at least one thyristor. Further, the present invention may applyto a composite bridge in which the respective positions of the diodesand thyristors are opposite with respect to the bridges of FIGS. 1 and9. Adapting the control circuit is within the abilities of those skilledin the art based on the functional indications given hereabove.

[0087] Such alterations, modifications, and improvements are intended tobe part of this disclosure, and are intended to be within the spirit andthe scope of the present invention. Accordingly, the foregoingdescription is by way of example only and is not intended to belimiting. The present invention is limited only as defined in thefollowing claims and the equivalents thereto.

What is claimed is:
 1. A method for controlling at least one thyristor(TH) constitutive of a rectifying bridge with a filtered output,comprising: closing the thyristor (TH) when the voltage thereacrossbecomes greater than zero; and making the gate current of the thyristordisappear when the current therein exceeds its latching current.
 2. Themethod of claim 1, wherein the voltage across the thyristor (TH) ismeasured by a unidirectional resistive rectifying bridge (R1-R2).
 3. Themethod of claim 1, wherein the latching current in the thyristor (TH) isdetected by measuring the voltage thereacross.
 4. A circuit forcontrolling at least one thyristor (TH) constitutive of a rectifyingbridge with a filtered output, comprising: a first comparator (121) forcontrolling a circuit providing a gate current to the thyristor, saidcomparator detecting that the voltage across the thyristor becomespositive; and an element for inhibiting the gate current circuit as soonas a current in the thyristor is greater than its latching current. 5.The circuit of claim 4, wherein said first comparator comprises a firstinput which receives the midpoint of a resistive dividing bridge (R1-R2)having its terminals connected, via a diode (D), to the terminals (A, K)of the thyristor (TH), and a second input which receives a firstreference voltage (Vref1, VBEN).
 6. The circuit of claim 4, wherein saidfirst comparator comprises a first bipolar transistor (T2), thebase-emitter voltage drop (VBEN) of which conditions said firstreference voltage.
 7. The circuit of claim 4, wherein the gate currentcircuit is formed of a constant current source (10; D3, D4, T1, R4)controlled by a switch (K, T1) connected to the gate (G) of thethyristor (TH).
 8. The circuit of claim 7, wherein said first comparatorcomprises a first bipolar transistor (T2), the base-emitter voltage drop(VBEN) of which conditions said first reference voltage, and wherein thegate current circuit comprises a second bipolar transistor (T1) havingits base connected to the collector of the first transistor (T2), theemitter of the second transistor being connected to a terminal ofapplication of a D.C. supply voltage (Vcc) via a resistor (R4) and itsbase being connected to this D.C. supply voltage by two diodes (D3, D4)in series.
 9. The circuit of claim 5, comprising: a second comparator(131) having an input receiving a voltage proportional to the current inthe thyristor (TH) and a second input receiving a second referencevoltage (Vref2); and a flip-flop (11), the respective set (S) and reset(R) inputs of which receive the outputs of the first and secondcomparators, and the output (O) of which is connected to a switch (K)for providing a gate current to the thyristor.
 10. The control circuitof claim 5, controlling several thyristors (TH1, TH2).
 11. Acontrollable rectifying bridge comprising at least one thyristor (TH1,TH2), comprising the control circuit of claim 5.